Parent Effectiveness Training is an old favorite book of mine. I have never taken the course or had an instructor (which is a neat thought), but I do think you can get a lot simply from the book.
Yes, Alfie Kohn does quote Tom Gordon in Unconditional Parenting. (I think maybe he interviewed him? And quotes from or references this interview in UP? Maybe the full interview is available someplace. Or maybe my memory is faulty!)
I think the similarities between NVC and P.E.T. are real and make sense, given the backgrounds of both authors. You could say that those similarities express the connection both authors had to Carl Rogers. Marshall Rosenberg & Thomas Gordon both were students of Carl Rogers (I am thinking in different institutions and at different times, but I don't know for sure.) I see Rogers and his values and assertions all over the work of both men. For me, what Carl Rogers said about how young children internalize "conditions of worth" gave me a strong sense of what to avoid as a parent, or how TO parent.
I think the way P.E.T. is constructed or organized, and laid out, could be especially appealing to some parents. I wonder if men who resist the idea of reading "parenting" books might like this one. I have always had the sense that my husband would like it a lot (he hasn't read it but has heard a lot about it!)
Mary934 mentioned the "I messages" and I believe this is a strong, emotionally responsible way of speaking (and thinking.) Speaking personally and owning your feelings in this way can help to transform interactions in relationships. I think it was a "missing piece" for me.
I recently was glancing at some of Gordon's comments about "active listening" and how he talks about translating or "decoding" a child's communication. I tend to think of it as reflective listening and as a way of bringing acceptance to communications that might be triggering or upsetting. And when you "reflect" you are translating that more primitive communication, thereby modeling what you would wish to hear from your upset child. But it certainly IS active listening, in that I actually am giving my child "herself," which facilitates a process (in her) that more passive listening/accepting does not quite do. It's not a matter of instructing (or being active in the sense of "guiding"), but rather very active listening and understanding, and reflecting back.
And I think if someone is struggling with the idea (or the "how?") of personal limits, P.E.T. is worth a read.